At a general level, marine radar systems typically comprise a rotating radar antenna scanner that transmits electromagnetic pulses during rotational 360° sweeps (scans) at a number of azimuthal directions in the sweep, the azimuthal sampling rate defined by the azimuthal resolution. For each pulse in an azimuthal direction the echo return signal is received and sampled at a number ranges from the scanner. The resultant digital radar video stream generated by the scanner is then typically signal processed to detect targets for presenting on a radar display, such as a Plan Position Indicator (PPI). The signal processing typically includes Sensitivity Time Control (STC) processing for detecting targets in the presence of clutter, such as echo returns caused by sea and rain clutter. STC typically provides a decaying echo return intensity threshold that is dependent on range. Echo returns in the digital radar video stream that exceed their corresponding STC threshold register as targets for display, while those below the threshold are characterized as clutter and ignored. In most modern marine radar systems, the STC threshold profile can be configured either manually or automatically, and can be customised based on developed models for the application or conditions based on various selectable input parameters including, but not limited to, such desired overall sensitivity (gain), rain state, and sea state.
Antenna sidelobe clutter is caused by echo returns from targets situated outside of the main beam or mainlobe of the antenna, but within a sidelobe beam zone of the antenna. In marine radar, sidelobe clutter exhibits as an arc of varying length centered around very large single targets and in the more detrimental case as regions of increased sidelobe clutter false alarms due to adjacent range and azimuth extensive target areas, such as land, built-up waterfront areas, bridges and the like. The occurrence of nuisance levels of sidelobe clutter is increased in small form factor radar systems due to lower azimuth sidelobe performance induced by the limited antenna horizontal size. At X-band marine frequencies, small form factor is generally taken to be antenna sizes of less than 600 mm and this includes the majority of all leisure marine radome scanner products. Also, larger antenna sizes of mainly open array types up to 1200 mm also suffer nuisance sidelobe clutter levels, but at a somewhat reduced level.
Sidelobe clutter is problematic for both manual and automatic STC as the sidelobe peaks are in fact true targets observed off the main beam due to the size of the source target. The current method of addressing such sidelobe clutter is to reduce sensitivity at all ranges and azimuths. This of course reduces the occurrence of sidelobe clutter, but at the expense of general sensitivity at ranges and bearings not affected by this type of clutter in the target scenario.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.
It is an object of the invention to provide an improved system and method for sidelobe clutter suppression in radar, or to at least provide the public with a useful choice.